1. Field of the Invention
This invention relates to a technique for producing and repairing reticles and photomasks useful in the fabrication of integrated circuits and related devices. More particularly, this invention in one embodiment relates to a technique for producing defect-free photomasks.
2. Description of the Prior Art
Photosensitive resist materials play an important part in the production of monolithic circuits. The use of such photoresist materials is based on their capability in permitting the "engraving" of circuit patterns of specific dimensions in a predetermined monolithic substrate material, as for instance silicon. This is effected by means of a photolithographic process where a two-dimensional pattern corresponding to the circuit design is first imaged on the photoresist coated substrate surface by employing a suitable exposure mask. Through a subsequent developing process, the desired resist patterns are obtained on the substrate surface. In this process, the photoresist serves as a protective system for those areas of the substrate surface that have not been bared by the preceding photolithographic process.
According to their interaction with light, photoresist systems are classified into negative-working and positive-working systems. A negative-working photoresist is one which after exposure is insoluble in a (developing) solvent, whereas the unexposed resist areas are dissolved by the "developer." As a result bared, unprotected areas are obtained on the substrate surface which correspond to the opaque or dark areas on the photomask. In a positive-working resist the photoresist is altered under exposure in such a manner that it is subsequently soluble in the developer. The exposed areas of the resist film are removed during developing, and the bared unprotected areas on the substrate surface correspond to the transparent areas on the photomask. As can be readily appreciated, in either the negative-working or positive-working microphotolithographic systems, the quality of the mask is of vital importance. The mask, like a photographic negative, (or positive) will duplicate any flaw or defect onto the photosensitive resist film.
Defects that reduce the quality of the mask and their yields can be devided into two major categories. The first category is that of opaque defects. These defects obstruct the passage of light preventing the exposure of the photoresist. There have been several methods applied to deal with opaque defects. One such process is to isolate the defect with the aid of a photoresist coating. The resist film protects the adjacent geometries leaving the defect exposed. The defect is then subjected to an etchant that removes it from the mask. Another method employs a laser beam focused onto the defect area, striking the defect with sufficient energy to vaporize it from the substrate. These techniques are commonly used and are adequate in their function. The other major category of defects, that of clear defects, occurs when the geometric pattern of the mask has been interrupted because of loss of material. This allows the exposure of the photoresist in undesired areas.
In an attempt to repair these clear defects, a variety of opaquing techniques have been attempted. In one such process an operator manipulating a tool such as a fine paint brush under a microscope, attempted to apply an opaquing fluid to the damaged part of the mask. This technique is time consuming and has limited application. A further process is one of deposition of a chrome film onto the missing pattern. This is a lengthy and involved procedure that gives inconsistent results.